Synopsis · Cell chemistry
Iron centers, iodine atoms and what an EU sentence really states
Here we dig further than on the home page: the proteins involved are sorted into tables, terms such as NADH or cytochrome are unpacked, and questions sent to us on this topic get answered.
Glucose, fatty acids and many amino acids start out on breakdown routes of their own. Most of them, however, finish at one shared intermediate: an activated acetic acid unit linked to coenzyme A. From this point on, all three nutrient groups run through identical machinery.
Within the mitochondria, acetyl-CoA is channeled into a loop of eight reactions called the citric acid cycle. Carbon exits the loop as carbon dioxide. What matters more for making sense of it is what does not leave as gas: electrons rich in energy.
Two coenzymes, NAD+ and FAD, catch the electrons and in doing so turn into NADH and FADH2. These carriers unload their freight onto the protein complexes of the inner mitochondrial membrane. There, the flow of electrons drives protons across the membrane; as the protons stream back, ATP synthase assembles ATP.
Almost no step of the electron handoff gets by without iron. The table sums up the chemical form the element takes in each individual complex.
| Protein or complex | Kind of iron center | What it does |
|---|---|---|
| Complex I (NADH dehydrogenase) | several iron-sulfur clusters in series | accepts electrons from NADH |
| Complex II (succinate dehydrogenase) | [2Fe-2S], [4Fe-4S], [3Fe-4S] plus heme b | links the citric acid cycle to the membrane chain |
| Complex III | cytochrome b, cytochrome c1, Rieske center [2Fe-2S] | hands electrons on to cytochrome c |
| Cytochrome c | a single heme group | shuttles between complex III and IV |
| Complex IV | heme a and heme a3 | last electron handover |
| Aconitase (citric acid cycle) | [4Fe-4S] | turns citrate into isocitrate |
This protein, named for the biochemist John S. Rieske, holds a [2Fe-2S] center in which one of the two iron atoms is, unusually, attached to two histidine residues rather than to cysteine. That attachment shifts its redox behavior in a way that lets it take over electrons from ubiquinol in a targeted manner.
With a little over a hundred amino acids, cytochrome c counts among the smallest proteins in the chain. Rather than being anchored in the membrane, it slides along the outer face and ferries one electron at a time from a complex to the next one. Meanwhile its iron atom alternates between Fe3+ and Fe2+.
Aconitase is an exception: its [4Fe-4S] cluster relays no electrons at all, but instead helps pull a water molecule off citrate and attach it again. Should the cube lose one of its four iron atoms, the enzyme stops working – a clear illustration of how directly single atoms shape the course of a metabolic pathway.
“Iron contributes to normal energy-yielding metabolism”
EU-authorized wording · Regulation (EU) No 432/2012
For iodine, the link sits one tier up. The element is not part of the membrane chain proteins as such; it resides in signaling molecules that decide how many of those proteins a cell produces to begin with.
| Compound | Iodine atoms per molecule | Remark |
|---|---|---|
| Iodide (I−) | 1 | the form in which most iodine is taken in |
| Monoiodotyrosine / diiodotyrosine | 1 / 2 | precursors, still attached to a carrier protein |
| Tetraiodothyronine (T4) | 4 | iodine makes up about 65% of the molecular mass |
| Triiodothyronine (T3) | 3 | binds to receptors in the cell nucleus |
| reverse T3 (rT3) | 3 | atoms arranged differently, barely active at the receptor |
In many tissues, enzymes known as deiodinases convert T4 into T3. Which of the four iodine atoms gets removed settles whether the product is a molecule that binds tightly to the receptor or the barely binding rT3. So here, the location of one atom sets the function.
T3 settles onto nuclear receptors that are coupled straight to the DNA. From that position, the frequency at which certain stretches are read is determined. The genes concerned include those for subunits of the respiratory chain complexes, for ATP synthase and for sodium-potassium ATPase. By this route, the oxygen a resting cell consumes is tied to how much iodine it receives.
“Iodine contributes to normal energy-yielding metabolism”
EU-authorized wording · Regulation (EU) No 432/2012
On EU labels, amounts are stated against the nutrient reference values (NRVs) set by Regulation (EU) No 1169/2011. For adults, these EU figures are 14 mg of iron per day and, separately, 150 µg of iodine per day. They are labeling benchmarks, not personal advice.
| Kind of food | of interest for | Good to know |
|---|---|---|
| Meat, organ meats | iron | provide heme iron, which is taken up comparatively well |
| Legumes, whole grains | iron | non-heme iron; vitamin C eaten in the same meal raises absorption |
| Ocean fish, seafood | iodine | amount differs by species and where it was caught |
| Iodized table salt | iodine | optional in many EU countries, quantity shown on the pack |
“Iron contributes to normal energy-yielding metabolism”
EU-authorized wording · Regulation (EU) No 432/2012
“Iodine contributes to normal energy-yielding metabolism”
EU-authorized wording · Regulation (EU) No 432/2012
No. The authorized wording describes the usual course of events when intake is sufficient. Once the iron centers of the enzymes are filled, extra iron does not add to their number. An excess can actually be unwanted, and that is why labels give an upper amount.
Signaling molecules that contain iodine have a say in how many membrane chain enzymes a cell makes. Iodine is not built into those enzymes itself; its part lies in controlling how genes are read, as the section on iodothyronines sets out.
In chemical terms, both eventually end up in the same storage and working forms. The path there is what differs: heme iron is absorbed in the small intestine by a dedicated mechanism, whereas non-heme iron relies more on what else the meal contains.
No. The legal text holds nothing but the short sentences. Cytochromes, iron-sulfur clusters and nuclear receptors come from the biochemical literature that EFSA drew on for its assessment. We describe them so the wording makes sense – not to stretch it.
The long-form edition of this page: more tables, structural drawings of the iron-sulfur clusters and a glossary running from acetyl-CoA to ubiquinone.
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